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Viral Turtles

A double stranded RNA (dsRNA) viral genome, introduced into a host cell, is met by formidable host defenses. The very presence of dsRNA in a eukaryotic or prokaryotic cell announces a viral infection and elicits effective responses, ranging from silencing of the viral mRNAs to apoptosis. Despite that, there are successful dsRNA viruses throughout the biosphere. By 2000, eight families with close to 200 "species" were known to infect bacteria, fungi, plants, and animals. The broad diversity of their hosts notwithstanding, all dsRNA viruses share the same secret to success: they bring their capsid into the cell along with their genome to serve as a safe compartment where they transcribe and replicate their genome. Their dsRNA is never exposed.

For the virus whose virion is but a simple protein capsid, it is the entire virion that enters and persists intracellularly. For others that have additional outer layers of protein and/or a membrane, those layers are removed during cell entry and the inner capsid alone enters the cytoplasm. The outer layers vary greatly from group to group, presumably reflecting adaptations to particular hosts or modes of transmission, while the proteins of the inner capsid, as well as its architecture, are highly conserved among all dsRNA viruses. Doesn't this suggest a common ancestry?

This strategy poses particular challenges, not the least of which is how do you transport something as large as a virion across the cell membrane. Also, since a dsRNA genome is not a suitable template for protein translation or for cellular replicases, these viruses have to bring their own RNA-dependent RNA polymerase (RdRp) with them. The capsid itself has to be selectively porous, allowing nucleotides to enter and RNA transcripts to exit.